1. Field of the Invention
The invention relates to a semiconductor device having a semiconductor body comprising a surface-adjoining semiconductor layer of a first conductivity type which is situated on a substrate region of the second opposite conductivity type and forms a p-n junction therewith, a separation region which extends from the surface over substantially the whole thickness of the semiconductor layer and surrounds an island-shaped region of the semiconductor layer, within which island-shaped region are present an active zone of the second conductivity type belonging to a semiconductor circuit element and a juxtaposed contact zone of the first conductivity type having a higher doping concentration than the semiconductor layer, the active zone and the contact zone both adjoining the surface and at least the contact zone being further entirely surrounded by the island-shaped region, the thickness and the doping concentration of the island-shaped region being so small that upon applying a voltage in the reverse direction across the p-n junction, the depletion zone extends up to the surface at a voltage which is lower than the breakdown voltage of the p-n junction.
It is to be noted that in the operating condition the same reverse voltage need not be present at any point across the p-n junction as a result of currents flowing parallel to the surface. As a result of the voltage drop caused by said currents it may occur that in the operating condition the island-shaped region is depleted in places where said reverse voltage is high and is not depleted entirely up to the surface in places where the reverse voltage across the p-n junction is comparatively low.
2. Description of the Prior Art
A semiconductor device of the kind described is disclosed in U.S. Pat. No. 4,292,642, assigned to the assignee of this application. A device is described in said patent wherein the island-shaped region constitutes the collector zone and the active zone of the second conductivity type constitutes the base zone of a vertical bipolar transistor. The emitter zone of the transistor is formed by a surface zone of the first conductivity type provided in said active zone.
As is explained in the above-mentioned patent, such a device has the important advantage that the breakdown voltage between the contact zone and, on the one hand the substrate region and on the other hand the active zone which in the operating condition often is at substantially the same potential as the substrate, may be very high and may even approach the unidimensionally calculated theoretical value. This results from the fact that at high collector-base voltage the island-shaped region is depleted up to the surface so that the field strength at the surface is considerably reduced.
Such a semiconductor device may be considered to be built up from a semiconductor circuit element with a juxtaposed junction field effect transistor therewith in series, the control electrode of which is formed by the substrate region.
A disadvantage of these devices is that the current which flows from the contact zone via the semiconductor layer (generally an expitaxial layer) to the active zone of the second conductivity type (or conversely, dependent on the conductivity types of the various regions), in the comparatively thin and high-ohmic part of the island-shaped region between the said active zone and the substrate region causes, at comparatively low values, a voltage drop which adversely influences the electrical properties of the device (the so-called "Kirk" effect, see IRE Transactions on Electron Devices, ED9, 1962 pp. 164-174). In the above-described bipolar transistor a considerable decrease of the gain (h.sub.FE) occurs with very low currents. In such a transistor the reduction of the series resistance between contact zone and base zone by a buried layer which is usual in conventional bipolar transistors and which layer extends from below the base zone to below the contact zone, is not possible since as a result of this the depletion zone in practice would be restricted to the buried layer and could not extend up to the surface so that the high breakdown voltages in view could no longer be reached.